Relationships between Rubisco total activity and mass-based nitrogen concentration (a) and between PEP carboxylase total activity and mass-based nitrogen concentration (b) of leaves (black-filled circles) and current-year stems (open circles) of Fagus sylvatica L. Each point corresponds to the mean value of a N treatment and mean value of enzyme activity (n = 3 for each ones). The standard errors of the ordinate and abscissa values are shown as vertical and horizontal bars, respectively.